(1) Field of the Invention
The present invention relates to heat exchanger tubes for heat exchangers used in chilling fluids or making ice. Specifically, the invention relates to an improvement in the heat exchangers where a bimetallic heat exchanger tube is used to increase the capacity of the heat exchanger and lower the rate of wear of other parts within the heat exchanger.
(2) Description of the Related Art
Heat exchangers for making a chilled fluid or water-ice mixture are well known in the art. These heat exchangers are used in a variety of industries including the food processing industry where a slurry ice mixture is used to refrigerate meats, produce, and fish. The heat exchangers are also used in the processing of milk products and concentrated fruit juices, and in HVAC and brewing applications. In these heat exchangers, the ice water mixture is pumped through heat exchanger tubes and refrigerant is circulated around the tubes. The fluid mixture is chilled as it passes through the, heat exchanger tubes, and the chilled mixture is pooled in a reservoir where it may be accumulated for further processing, as required. In order to prevent ice from forming in the tube and adhering to the tube, mechanical agitators are used to maintain the flow of the chilled mixture through the heat exchanger.
One machine that has gained wide use in the food processing industry is a whip rod heat exchanger. A typical whip rod heat exchanger has vertical heat transfer tubes with whip rods positioned in the heat transfer tubes. Feed or process solution is directed into the tubes. A drive mechanism in the heat exchanger develops relative motion between the tubes and the whip rods to distribute the feed solution evenly within the tube, thus more effectively freezing or chilling the feed solution. An orbital tube evaporator system is described in U.S. Pat. No. 5,221,439, issued Jun. 22, 1993, and entitled Orbital Tube Evaporator With Improved Heat Transfer. An improvement to that apparatus is described in U.S. Pat. No. 5,385,645, issued Jan. 31, 1995, and entitled Heat Transfer Apparatus with Positive Drive Orbital Whip Rod, the disclosures of which are incorporated herein by reference. In the orbital whip rod system described in the '645 patent, the feed solution is pumped through the vertical tubes and chilled by the refrigerant circulating in the chamber surrounding the vertical heat exchanger tubes. The whip rod moves in an orbital direction inside a stationary heat exchanger tube so that the fluid is distributed as a film on the inner diameter surface of the tube. The whip rod creates a turbulent flow liquid layer that prevents the feed solution from sticking to the inner wall of the tube as it is chilled and moves from the top of the heat exchanger to the bottom of the heat exchanger. The refrigerant is circulated around the outside of the heat transfer tube to remove heat from the feed solution to chill the feed solution. The chilled feed solution may then be directed from the bottom of the vertical tubes to a storage tank where it may be pumped away or otherwise utilized as required by the application.
An improvement to the orbital whip rod heat exchanger is described in U.S. Pat. No. 5,953,924, issued Sep. 21, 1999, and entitled Apparatus, Process and System for Tube and Whip Rod Heat Exchanger, the disclosure of which is incorporated herein by reference. The '924 patent describes an invention where the orbital whip rod heat exchanger may be configured to operate in a flooded tube mode where the heat exchanger tubes are flooded with process fluid and the whip rod is driven from both ends of the heat exchanger tube, and a falling film mode, where the heat exchanger tube is partially filled with process fluid and the whip rod is suspended in the tube and driven from a drive plate above the tube.
Another style of heat exchanger for producing an ice-slurry mixture found in wide spread use in the food processing industry is a heat exchanger with flooded refrigerated tubes having a rotating blade inside the tube. These heat exchangers are described in U.S. Pat. No. 5,884,501 issued Mar. 23, 1999, and U.S. Pat. No. 6,056,046, issued May 2, 2000, the disclosures of which are hereby incorporated by reference. The ice slurry mixture is formed in a central tube in a heat exchanger housing and is moved from an inlet to an outlet in the central tube by a rotating blade. Refrigerant is circulated in refrigerant tubes formed around the outer periphery of the heat exchanger housing. The rotating blades move past the inner wall of the central tube of the heat exchanger without contacting it and thereby move cooled fluid from the surface to prevent the deposition of ice crystals on the inner wall of the heat exchanger.
In these conventional heat exchangers, including the orbital whip rod heat exchanger, the tubes are generally made from stainless steel since stainless steel is both corrosion resistant to the feed solution being pumped through the inner wall surfaces of the tube and the refrigerant which circulates around the outer wall surfaces of the heat exchanger tube. Stainless steel also has good thermal conductivity and good strength. In stainless steel tube heat exchangers, HCFC and ammonia may be used as refrigerants in a variety of applications where the chilled feed solutions may include seawater and food products. Copper has also been used for constructing the heat exchanger tubes; however, copper is generally not compatible with ammonia based refrigerant systems. In industries where there is a higher concern for safety, HCFC refrigerants continue to be used. On the other hand, in process cooling applications, ammonia based refrigerants are well suited because of their low cost and high efficiency. As stainless steel tubes can be used in both the systems, stainless steel heat exchanger tube systems are common.
However, stainless steel heat exchanger tube systems have many disadvantages. Stainless steel tube heat exchangers are generally more expensive when compared to copper tube heat exchangers. In the construction of copper tube or stainless steel tube heat exchangers, the tubes are generally roll fastened to the tube sheet. The tube to tube sheet connection must be a leak tight boundary to prevent the refrigerant from leaking out of the heat exchanger unit and into the feed solution and possibly the surrounding workspaces. Since ammonia is toxic, in some applications that use ammonia as a refrigerant, the stainless steel tube must be welded to the tube sheet in addition to being roll fastened to ensure the required leak tight boundary for the application. Since copper tube exchangers do not use ammonia, the tube to tube sheet connection need only be roll fastened, thus lowering manufacturing cost. Stainless steel also generally has a higher material cost than copper.
In these types of heat exchangers, seamless stainless steel tube is preferred. A welded tube may also be used if the weld bead is flattened to provide a smooth transition over the seam. In the orbital rod heat exchanger, the smooth inner surface of the tube allows the whip rod to travel along the inner surface without "bumping" or jumping over the seam. The consistent motion of the whip rod creates the turbulent flow layer needed to prevent ice formation in the tube. In the blade type heat exchanger, the smooth surface is required so that the small radial clearance between the blades and the inner surface of the tube is maintained for fluid flow along the inner surface of the tube. A more costly seamless tube or the secondary operation of flattening the weld bead is an added expense. Since copper tubes are usually drawn without a seam when manufactured, they are generally less expensive than seamless stainless steel tubing.
Stainless steel heat exchanger tubes also have other drawbacks when compared to conventional copper-based systems. In order to depress the freezing point of solution as it is processed and promote the formation of ice slurry, additives such as ethylene glycol, propylene glycol, urea, and ethanol may be added. These substances when used in the feed solution cause the ice crystals of the feed solution to form as small flakes or as powdery solids rather than large, flat flaky crystal structures. The powdery consistency of the ice particles prevents the feed solution from adhering to the inner wall surfaces of the heat exchanger tube. Other corrosion inhibiting substances such as di-potassium phosphate (K.sub.2 HPO.sub.4) are also commonly added.
Although the additives enhance the formation of the ice slurry mixture in the tubes, the use of these additives in the feed solution tends to accelerate corrosion and wear of components in the heat exchanger. Specifically, the eccentrics and tube inserts in the drive mechanism for positioning and driving the whip rod in the tube are made from hardened steel materials. This difference in materials in the presence of the feed solution accelerates corrosive attack on these drive components. In an orbital rod heat exchanger having a stainless steel tube construction, the tube inserts and eccentrics are most anodic. Because these parts generally have a small surface area, they are highly susceptible to galvanic corrosion. As tube inserts and eccentrics corrode, their surfaces grow rough, and the hardened, roughened surfaces contribute to accelerated wear of the other components in the drive mechanism such as the plastic counter crank, plastic sleeve bearing, and whip rod.
The attack tends to be more acute when the concentration of corrosion inhibiting agents in the feed solution is low. To slow the corrosion, the concentration of the additives must be maintained relatively high. Generally, phosphate must be maintained at a level of at least 1000 ppm, and preferably 4000 ppm, in order to prevent corrosion. The level of additives must be periodically monitored to prevent deterioration of the system. Additionally, certain areas in the drive system experience a combination of corrosion and erosion from suspended solids in the feed solution. In order to prevent this type of rapid wear, secondary filtration system are commonly installed on ice making machines with stainless steel heat exchanger tubes. This also adds cost to the machine.
In addition to the higher manufacturing costs and wear problems found with stainless steel tube heat exchangers, ice making machines using stainless steel tubes generally have a lower ice making capacity when compared to ice making machines with heat exchangers using copper tubes.